Cathode heater testing circuit for a vacuum tube network



Aug. 13, 1940. R H. HERRICK 2,211,421

CATHODE HEATER TESTING CIRCUIT FOR A VACUUM TUBE NETWORK Filed Jan. 10,1938 1 C I j T "W n m INVENTOR. L3 i ROSWELL H.HERRICK BY M M ATTORNEYfiPatented Aug. 13, 1940 UATHODE HEATER TESTING CIRCUIT FOR A VACUUIW TUBENETWORK Roswell H. Herrick, Oak Park,

111., assignor to Associated Electric Laboratories, Inc., Chicago,

111., a. corporation of Delaware Application January 10, 1938, SerialNo. 184,254

8 Claims.

The present invention relates to vacuum tube networks and moreparticularly to improvements in circuits for determining the conditionof vacuum tubes embodied in signal amplifiers and the In certainapplications such, for example, as small communication systems, it isadvantageous from the standpoint of economy of construction andoperation to use an available current source for a number of differentpurposes. Ordinarily, the voltage of such a source is too high to permitthe connection of vacuum tube heaters directly in parallel therewith.Hence, when a vacuum tube network forms a portion of the apparatus to beenergized from the source, it becomes desirable to connect the cathodeheaters of the respective tubes in series with each other and with acurrent determining resistor across the source. When the seriesarrangement is used, however, it is quite difiicult to locate a tubehaving an opencircuited cathode heater.

It is an object of the present invention to provide improved andexceedingly simple apparatus for quickly determining the condition ofthe '25 cathode heaters embodied in the vacuum tubes of a vacuum tubenetwork.

More specifically, it is an object of the inven tion to provide improvedapparatus for rapidly locating a tube having an open-circuited cathodeheater in a vacuum tube network wherein the cathode heaters areconnected in series across a current supply source.

The invention as described in detail hereinafter is illustrated in itsembodiment in a signal amplifier of the general form used in publicaddress systems. In brief, the objects as set forth above are attainedin accordance with the present invention by providing a test orindicating lamp, and means comprising a manually operable ro- 10 taryswitch for selectively connecting the indicating lamp in parallel withany one of the cathode heaters respectively embodied in the vacuum tubesof the amplifier. More particularly, the heater selecting switchcomprises two sets of contacts, and two uni-controlled wipers having theindicating lamp connected therebetween and each associated with one ofthe contact sets. Circuit connections are provided which extend betweenthe contacts of the switch and the heater terminals and are so arrangedthat the indicating lamp is successively connected in parallel withdifierent ones of the series connected oathode heaters as the wipers ofthe switch are operated successively to engage different ones of their 3associated contacts. The voltage rating of the indicating lamp is chosensubstantially to equal the voltage of the source and, hence, itsresistance is relatively high as compared to that of each of the cathodeheaters.

By the arrangement de scribed, and when the lamp is connected inparallel with the cathode heater of a selected tube, it is onlyenergized to radiate its rated illuminating power when the parallelconnected heater is opencircuited.

The novel features believed to be characteristic of the invention areset forth with particularity in the appended claims.

The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the specification taken in connection withthe accompanying drawing, in which there is illustrated a signalamplifier having incorporated,

therein the features of the present invention.

Referring now to the drawing, the improved signal amplifier isillustrated as comprising three amplifier stages In, H

and I2 coupled in tandem in the order of their enumeration and arrangedto amplify signal voltages impressed between the amplifier as indicatedand to supply the amplified output loud speaker or translating device"5. More particua three-elechaving its input electrodes l8 andl9 coupledto the input terminals I 3 and I5 by means of a transformer 20, and itsoutput electrodes I9 and 2| coupled by means of an output circuitincluding a resistor 22 to the input circuit 23 of a secondthree-electrode vacuum tube 24 included in the second amplifier stageII. The control grid in the input circuit of serially connected gridbiasing cells 25 biasing network provided the tube l1 comprises two and26,

which are connected between the cathode l9 and the lower terminal of thesecondary winding of the transformer 20, and are by-passed for signalfrequency currents by a capacitor 27.

In order to balance the primary winding 01 the transformer 20 withrespect to ground, thereby to minimize stray coupling to the inputcircult of the tube l1, there are connected in series across thiswinding two resistors 8 and 9 having the junction point therebetweengrounded as indicated. The cathodes of the tubes I! and 24 aremaintained substantially at ground poten-. tial, as regards signalcurrents, through the provision of a coupling condenser 1 connected asshown. The coupling network connected between the two amplifier stagesII and I 2 comprises a transformer 28 having a primary winding 29forming a part of the output circuit of the tube 24 and a secondarywinding 30 comprising a portion of the input circuit for the push-pullconnected tubes 3| and 32 of the power stage I2. Each of the twolast-mentioned tubes is of the well-known pentode type comprising ananode, a cathode, a control grid, a suppressor grid and a screen grid.The two control grids of the tubes 3| and 32 are respectively connectedto opposite terminals of the secondary winding 30 and the cathodesthereof are connected together and to the center point of the winding 30through a cathode biasing resistor 33 which functions in a well-knownmanner to maintain the proper potential relations between the inputelectrodes of each of the two tubes noted. Also in accordance withconventional practice the suppressor grids of the tubes 3| and 32 aremaintained at cathode potential through the connections indicated in thedrawing. The respective anodes of tubes 3| and 32 are connected toopposite terminals of the primary winding 33' of a transformer 3 havingits secondary winding 35 connected to the input terminals of the loudspeaker IS. The midpoint of the primary winding 33 is connected to eachof the screen grids included in the two tubes 3| and 32.

Anode potential is supplied to the anodes of the tubes H, 24, 3| and 32and to the screen grids of the tubes 3| and 32 from a supply source l4.A filter network indicated generally at 3B is provided for preventingtransient changes in the voltage of the source M from introducing noisein the amplifier channel. Specifically, this network comprises a filtercircuit including a choke inductance element 38 and two capicitors 35and d connected between the source it and the lower terminal of theresistor 22, and a similar filter circuit including the choke inductanceelement 4|, a capacitor 12 and the capacitor 50 connected between thesource l4 and the anode of the tube 24. The last-described filtercircuit is also connected between the source It and the screen and anodeelectrodes of the tubes 3! and 32.

As illustrated in the drawing, each of the tubes embodied in theamplifier is of the indirectly heated cathode type, the heaters of whichare connected in series across the voltage source This series circuitalso includes a circuitinterrupting switch 6| and a current limitingresistor 00 for maintaining the current flowing through the circuit atthe rated current value of the tube heaters. As pointed out in theintroductory portion of the specification, when the plurality of tubeheaters are energized in series it is quite diificult to locate in ashort time a tube having an open-circuited heater. Accordingly, there isprovided, in accordance with the present invention, a novel heatertesting unit indicated generally at 62 for quickly locating a tuberendered defective by a burned out or broken cathode heater.

Considering now the system as a whole and disregarding for the presentthe details of the input circuit 23 and the testing unit 62, it isbelieved that the operation of the amplifier as described above, willreadily be understood by those skilled in the art. In brief, a signalvoltage impressed between the input electrodes 13 and i5 is stepped upby the transformer 2.0 and impressed on the input electrodes of the tubell. The amplified voltage output of this tube is impressed by means ofthe input circuit 23 on the input electrodes of the tube 24, which tubefurther amplifies the voltage and delivers its output to the transformer28. Further voltage gain is obtained in the transformer 28 whichdelivers the driving voltage to the control grids of the tubes 3| and 32of the power stage i2. Finally, the amplified power output of the stageI2 is delivered to the input terminals of the loud speaker I6 whichconverts the signal currents into sound energy.

In certain applications the voltage of the source It is necessarilylimited to a relatively low value. In railroad train power systems, forexample, this voltage is usually of the order of sixty-five volts and,hence, must be utilized to the fullest extent if any considerablevoltage gain is to be realized in each of the driver stages l0 and II.To this end, the separate control grid biasing network comprising thecells 25 and 20 is used in the input circuit for the tube ll so thatsubstantially the entire voltage of the source I4 is efiectively appliedto the anode 2! of this tube. For the same reason independent biasingcells are used in the input circuit 23 for the tube I'l. The particulararrangement of this input circuit forms the subject matter disclosed andclaimed in applicants above referred to co-pending application. In orderto minimize trouble occasioned by deterioration of the biasing cellsincluded in the input circuits for the tubes l1 and 24, these cells arepreferably of the type known in the trade as The Mallory grid bias cell.These cells, which are manufactured by the P. R. Mallory 84 Company,Incorporated, are described in a descriptive bulletin issued by thiscompany June 15, 1935, and are known to have a life which exceeds theordinary life of any commercially available vacuum tube.

Briefly described, the circuit 23 comprises a voltage dividing resistor43 having its low potential terminal directly connected to the cathode44 of the tube 24 and its high potential terminal capacitively coupledby means of a capacitor 45 to the anode 2| of the tube N. This resistorhas an adjustable portion 46, the value of which is determined by thesetting of an adjustable terminal 4-1, which terminal is slidable alongthe resistor 03 to vary the signal voltage input to the tube 24 and,hence, the output volume of the loud speaker l6. Biasing potential isapplied to the control grid 48 of the tube 24 through the provision ofthree serially-connected cells 19, 50 and 5|, which are connectedbetween the control grid 48 and the adjustable terminal M of the voltagedividing resistor 00. For reasons pointed out above it is desirable touse cells 49. 50 and 5| of the Mallory type as the bias voltage source.Since, however, each of these cells has an internal resistance rangingfrom 11,000 to 50,000 ohms, it is impractical to connect the samebetween the cathode 44 and the control terminal of the resistor 43.Thus, if this were done, from 33,000 to 150,000 ohms of resistance wouldbe included in the adjustable portion of the resistor 03. This resistor,in the particular arrangement illustrated, is of approximately 500,000ohms resistance and, hence, if the biasing cells were connected in themanner described, the minimum signal voltage level attainable would belimited to from approximately six to twenty-two per cent of the levelfor maximum signal voltage input to the tube 24. It will be seen,therefore, that the presence of the control grid biasing cells in thecathode leg of the input circuit would definitely limit the minimumvoltage setting of the volume control signal voltage divider. With ofthe cathode heaters.

the arrangement illustrated, however, the adjustable terminal 41 of thevoltage dividing resistor may be moved to a point where the signalvoltage input to the input electrodes of the tube M is reduced to zero.

Turning now to a consideration of the testing unit 62, this unitcomprises, in general, an indicating device in the form of a lamp 63 andmeans comprising a manually operable rotary switch 64 for selectivelyconnecting the lamp 63 in parallel with any one of the tube heaters.More specifically, the switch 64 comprises two sets of contacts 65 and66 to which the terminals of the respective cathode heaters areconnected, and a pair of uni-controlled wipers 67 and 68, associatedrespectively, with. the contact sets 67 and 68 and having the lamp 63connected therebetween. The mode of connecting the heater terminals tothe contacts of the sets F5 and 66 is such that with the wipers of theswitch 64 cperated to any off-normal position, the lamp 6-3 is connectedin parallel with one of the cathode heaters. Thus, the oppositeterminals of, the cathode heater in the tube ll are connected,respectively, to the respective first contacts of the sets 65 and 66;the opposite terminals of the cathode heater in the tube 24 areconnected, respectively, to the respective second contacts of thecontact sets 65 and G5; the opposite terminals of the cathode heater inthe tube 3! are connected. respectively, to the respective thirdcontacts of the sets 65 and 65; and the opposite terminals of thecathode heater in the tube 32 are connected, respectively, to therespective fourth contacts of the sets 65 and 66. The fifth off-normalposition of the switch 64 is reserved as a test position for determiningthe condition of the indicating lamp 63. To this end, the fifth contactof the set 65 is adapted to be connected by way of a jumper conductor 63and the switch iii to the negative terminal of the source l4 and thefifth contact of the set 56 is connected by way of a resistor Ill andthe resistor lid to the positive terminal of the source. The resistancevalue of the resistor 11! is chosen to equal the resistance value ofthree of the cathode heaters connected in series, so that when thewipers of the switch 64 are operated to the test position the normalrated voltage of the lamp is impressed on the filament thereof. In orderto eliminate unnecessary wiring, jumper wires between the contacts areused wherever possible. For example, the circuit connections include aconductor ll con-- nected to the second and third contacts of the set 65by a jumper or multiple wire 72 and to the junction point between theconnected terminals of the heaters embodied in the tubes 24 and 3!.Similar jumper connections employed in the manner shown simplifythewiring to a considerable degree.

By providing the above described testing unit, it is possible rapidly tocheck the condition of any Thus, if' the amplifier suddenly stopsfunctioning the operator may actuate the wipers of the switch 64 to thetest position and if the lamp 63 lights up is informed that this lamp isoperating properly. Following this testing operation, the switch wipersmay successively be operated into engagement with different ones oftheir respective associated contacts. In each position, other than thefifth oiT- normal position, one of the cathode heaters is connected inparallel with the lamp 63, Now the filament of this lamp is of arelatively high resistance as compared to that of each ofrthe heatersand, hence, if a parallel connected heater is not open-circuited,insuflicient voltage is impressed on the lamp to cause any substantialillumination thereof. If, however, the lamp is connected in shunt withan open-circuited heater, the voltage of the source M, less the smallvoltage drops through the resistor iii] and the other heaters, isimpressed on the lamp filament to cause the lamp to assume its fullyilluminated condition. Such illumination of the lamp indicates an opencircuit in the heater of the tube corresponding to the particularposition to which the switch 64 is operated.

Although the invention has been illustrated in its application tocircuits for energizing the cathode heaters of tubes of the indirectlyheated cathode type, it will be understood that it is equally applicableto circuits for energizing cathodes of the directly heated or fil menttype wherein the electron emitting material forming the cathode iscoated directly on the heater element.

While there has been ent considered to be the of the invention, it willbe understood that various modifications may be made in the circuitarrangement, and it is contemplated in the appended claims to cover allsuch modifications as fall within the true spirit and scope of the invention.

I claim:

1. In a vacuum tube network, a plurality of vacuum tubes each comprisinga cathode heater, a source of heater current, means connecting saidheaters in series across said source, a manually operable switchcomprising two sets of bank contacts and two uni-controlled wipers eachassociated with one of said contact sets, an indicating lamp connectedbetween said wipers, and circuit connections extending between theterminals of said heaters and the contacts of said switch, said circuitconnections being so arranged that said indicating lamp is successivelyconnected in parallel with diiTerent ones of said heaters as said wipersare operated successively to engage different ones of their associatedcontacts, and said lamp having a relatively high resistance such that itis substantially illuminated only at the voltage of said source, wherebyan indication described what is at prespreferred embodiment is given bysaid lamp only when it is connected in parallel with an heaters.

2. In a vacuum tube network, a plurality of vacuum tubes each includinga cathode heater, a source of heater current, means connecting saidheaters in series across said source, a manually operable switchcomprising two sets of contacts and two uni-controlled wipers eachassociated with one of said contact sets, an indicating lamp connectedbetween said wipers, and circuit connections extending between theterminals of said heaters and the contacts of said switch, said circuitconnections including a conductor connected to two contacts of one ofsaid contact sets and to a junction connection between two heaterterminals, and being so arranged that said indicating lamp issuccessively connected in parallel with different ones of said heatersas said wipers are operated successively to engage different ones oftheir associated contacts,

3. In a vacuum tube network, a plurality of vacuum tubes each comprisinga cathode heater, a source of heating current, means connecting saidheaters in series across said source, a manually operable switchcomprising two sets of bank open-circuited one of said contacts and twouni-controlled wipers each associated with one of said contact sets, anindicating lamp connected. between said wipers, and circuit connectionsextending between the terminals of said heaters and the contacts of saidswitch, said circuit connections including a conductor connected to twocontacts of one of said contact sets and to a junction connectionbetween two heater terminals, and being so arranged that said indicatinglamp is successively connected in parallel with difierent ones of saidheaters as said wipers are operated successively to engage differentones of their associated contacts, and said lamp having a relativelyhigh resistance such that it is substantially illuminated only at thevoltage of said source, whereby an indication is given by said lamp onlywhen it is connected in parallel with an open-circuited one of saidheaters,

4. In a vacuum tube network, a plurality of vacuum tubes each comprisinga cathode heater a source of heater current, means connecting saidheaters in series across said source, a manually operable switchcomprising two sets of contacts and two uni-controlled wipers eachassociated with one of said contact sets, an indicating lamp connectedbetween said wipers, circuit connections extending between the terminalsof said heaters and the contacts of said switch, said circuitconnections being so arranged that said indicating lamp is successivelyconnected in parallel with different ones of said heaters as said wipersare operated successively to engage different ones of their associatedcontacts, and means including said wipers, the contact of one of saidsets and the corresponding contact of the other of said sets forconnecting said lamp across said source to test the condition thereof,said lamp having a relatively high resistance such that it issubstantially illuminated only at the voltage of said source, whereby anindication is given by said lamp only when it is connected across saidsource as in parallel with an open-circuited one of said heaters.

5. In an electrical network, a plurality of devices to be tested, saiddevices being susceptible of becoming open-circuited, a source ofcurrent, means connecting said devices in series across said source, amanually operable switch comprising two sets of contacts and twouni-controlled wipers each associated with one of said contact sets, anindicating device connected between said wipers and adapted to beenergized from said source when connected in parallel with an opencircuited one of said first-mentioned devices, and circuit connectionsextending between said first-mentioned devices and the contacts of saidswitch, said circuit connectionsincluding a conductor connected to twocontacts of one of said contact sets and to a junction connectionbetween the terminals of two of said first-mew tioned devices, and beingso arranged that said indicating device is successively connected inparallel with different ones of said first-mentioned devices as saidwipers are operated successively to engage different ones of theirassociated contacts.

6. In an electrical network, three devices to be tested, said devicesbeing susceptible of becoming open-circuited, a source of current, meansconnecting said devices in series across said source, a manuallyoperable switch comprising two sets of contacts and two uni-controlledwipers each associated with one of said contact sets, an indicatingdevice connected between said wipers and adapted to be energized fromsaid source when connected in parallel with an open circuited one ofsaid first-mentioned devices, a first con ductor extending from thejunction connection between the terminals of the first and second ofsaid first-mentioned devices to two adjacent contacts of one of saidcontact sets, a second conductor extending from the junction connectionbetween the terminals of the second and third of said devices to twoadjacent contacts of the other of said contact sets, the second contactof said last-mentioned two contacts being arranged to be engaged by itsassociated wiper simultaneously with the engagement of the secondcontact of said first-mentioned two contacts by its associated wiperduring operation of said switch, and circuit connections including saidconductors for successively connecting said indicating device inparallel with difierent ones of said firstmentioned devices as saidwipers are operated successively to engage different ones of theirassociated contacts.

7. In an electrical network, three devices to be tested, said devicesbeing susceptible of becoming open-circuited, a source of current, meansconnecting said devices in series across said source, a manuallyoperable switch comprising two sets of contacts and two uni-controlledwipers each associated with one of said contact sets, an

indicating device connected between said wipers andadapted to beenergized from said source when connected in parallel with an opencircuited one of said first-mentioned devices, a first conductorextending from the junction connection between the terminals of thefirst and second of said first-mentioned devices to two adjacentcontacts of one of said contact sets, a second conductor extending fromthe junction connection between the terminals of the second and third ofsaid devices to two adjacent contacts of the other of said contact sets,the second contact of said last-mentioned two contacts being arranged tobe engaged by its associated wiper simultaneously with the engagement ofthe second contact of said first-mentioned two contacts by itsassociated wiper during operation of said switch, circuit connectionsincluding said conductors for successive 1y connecting said indicatingdevice in parallel with difierent ones of said first-mentioned devicesas said wipers are operated successively to engage different ones oftheir associated contacts, and means including said wipers, the contactof one of said sets and the corresponding contact of the other of saidsets for connecting said indicating device across said source to testthe condition thereof,

8. In a vacuum tube network, a plurality of vacuum tubes each includinga cathode heater, a source of current, a, circuit serially includingsaid source said heaters and a resistor, a manually operable switchcomprising two sets of contacts and wipers individually associated with.said contact sets, an indicating lamp connected between said wipers,circuit connections extending between the terminals of said heaters andthe contacts of said switch, said circuit conductors including aconductor connected to two contacts of one of said contact sets and to ajunction connection between two heater terminals and a second conductorextending from the next succeeding junction connection between twoheater terminals to two contacts of the other of said contact sets, oneof said last-mentioned contacts corresponding to one of the two contactsof said one contact set, said circuit connections being so arranged thatsaid indicating lamp is successively connected in parallel withdifierent ones of said heaters as said wipers are operated successivelyto engage diiferent ones of their assohich is relatively much greaterthan the resistance of one of said heaters, whereby it is substantiallyilluminated only at the voltage or" said source and an indication isgiven by said lamp only when it is connected in parallel with anopen-circuited one of said heaters; and circuit means serially includingsaid resistor, an additional resistor having a resistance approximatingthe combined series resistance of said heaters, said wipers and a pairof corresponding contacts of said contact sets for connecting said lampacross said source to test the condition thereof. ROSWELL H. HERRICK.

